Human immunodeficiency virus (HIV) frequently causes neurological dysfunction and is abundantly expressed in the central nervous system (CNS) of acquired immunodeficiency syndrome (AIDS) patients with HIV encephalitis or myelopathy. The virus is found mostly in cells of the monocyte-macrophage lineage within the CNS, but the possibility of infection of other glial cells has been raised. Therefore, the effects of different HIV-1 and HIV-2 strains were studied in primary cultures of adult human brain containing microglial cells, the resident CNS macrophages, and astrocytes. These cultures could be productively infected with macrophage-adapted HIV-1 isolates but not with T lymphocyte-adapted HIV-1 isolates or two HIV-2 isolates. As determined with a triple-label procedure, primary astrocytes did not express HIV gag antigens and remained normal throughout the 3-week course of infection. In contrast, virus replicated in neighboring microglial cells, often leading to their cell fusion and death. The death of microglial cells, which normally serve immune functions in the CNS, may be a key factor in the pathogenesis of AIDS encephalitis or myelopathy.

Seawater-strength salt stress of the ice plant (Mesembryanthemum crystallinum) initially results in wilting, but full turgor is restored within approximately 2 days. We are interested in a mechanistic explanation for this behavior and, as a requisite for in-depth biochemical studies, have begun to analyze gene expression changes in roots coincident with the onset of stress. cDNAs that suggested changes in mRNA amount under stress were found; their deduced amino acid sequences share homologies with proteins of the Mip (major intrinsic protein) gene family and potentially encode aquaporins. One transcript, MipB, was found only in root RNA, whereas two other transcripts, MipA and MipC, were detected in roots and leaves. Transcript levels of MipB were of low abundance. All transcripts declined initially during salt stress but later recovered to at least prestress level. The most drastic decline was in MipA and MipC transcripts. MipA mRNA distribution in roots detected by in situ hybridization indicated that the transcript was present in all cells in the root tip. In the expansion zone of the root where vascular bundles differentiate, MipA transcript amounts were most abundant in the endodermis. In older roots, which had undergone secondary growth, MipA was highly expressed in cell layers surrounding individual xylem strands. MipA was also localized in leaf vascular tissue and, in lower amounts, in mesophyll cells. Transcripts for MipB seemed to be present exclusively in the tip of the root, in a zone before and possibly coincident with the development of a vascular system. MipA- and MipB-encoded proteins expressed in Xenopus oocytes led to increased water permeability. mRNA fluctuations of the most highly expressed MipA and MipC coincided with turgor changes in leaves under stress. As the leaves regained turgor, transcript levels of these water channel proteins increased.

K+ channels play diverse roles in mediating K+ transport and in modulating the membrane potential in higher plant cells during growth and development. Some of the diversity in K+ channel functions may arise from the regulated expression of multiple genes encoding different K+ channel polypeptides. Here we report the isolation of a nove1 Arabidopsis fhaliana cDNA (AKTZ) that is highly homologous t o the two previously identified K+ channel genes, KATl and AKT7. This cDNA mapped t o the center of chromosome 4 by restriction fragment length polymorphism analysis and was highly expressed i n leaves, whereas AKT7 was mainly expressed in roots. I n addition, we show that diversity in K+ channel function may be attributable t o differences i n expression levels. lncreasing KAT7 expression i n Xenopus oocytes by polyadenylation of the KATl mRNA increased the current amplitude and led t o higher levels of KATl protein, as assayed i n western blots. The increase in KATl expression in oocytes produced shifts in the threshold potentia1 for activation to more positive membrane potentials and decreased half-activation times. These results suggest that different levels of expression and tissue-specific expression of different K+ channel isoforms can contribute to the functional diversity of plant K+ channels. The identification of a highly expressed, leaf-specific K+ channel homolog in plants should allow further molecular characterization of K+ channel functions for physiological K+ transport processes in leaves.

SummaryThe cellular mechanisms that regulate potassium (K +) channels in guard calls have been the subject of recent research, as K + channel modulation has been suggested to contribute to stomatal movements. Patch clamp studies have been pursued on guard call protoplests of Vicia faba to analyze the effects of physiological cytosolic free Ca 2+ concentrations, Ca 2+ buffers and GTP-binding protein modulators on inward-rectifying K + channels. Ca 2+ inhibition of inward-rectifying K + currents depended strongly on the concentration and effectiveness of the Ca 2+ buffer used, indicating a large Ca 2+ buffering capacity and pH increases in guard calls. When the cytosolic Ca 2+ concentration was buffered to micromolar levels using BAPTA, inward-rectifying K + channels were strongly inhibited. However, when EGTA was used as the Ca 2+ buffer, much less inhibition was observed, even when pipette solutions contained 1 pM free Ca 2+. Under the imposed conditions, GTPTS did not significantly inhibit inward-rectifying K + channel currents when cytosolic Ca z+ was buffered to low levels or when using EGTA a~the Ca 2+ buffer. Furthermore, GDP~S reduced inward K + currents at low cytosolic Ca 2+, indicating a novel mode of inward K + channel regulation by G-protein modulators, which is opposite in effect to that from previous reports. On the other hand, when Ca 2+ was effectively elevated in the cytosol to 1 pM using BAPTA, GTP~S produced an additional inhibition of the inward-rectifying K + channel currents in a population of cells, indicating possible Ca2+-dependent action of GTPbinding protein modulators in K + channel inhibition. Assays of stomatal opening show that 90% inhibition of inward K + currents does not prohibit, but slows, stomatal opening and reduces stomatal apertures by only 34% after 2 h light exposure. These data suggest that limited K + channel down-regulation alone may not be rate-limiting, and it is proposed that the concerted action of protonpump inhibition and additional anion channel activation is likely required for inhibition of stomatal opening.